Flexible resistive sensors, such as flexure sensors, flex sensors, bending sensors, strain gages, etc., can be used to measure various conditions such as temperature, moisture, air flow, mechanical stress, etc. A variable resistance element can be provided on a flexible substrate that changes shape and/or dimensions based on the condition being measured. More specifically, an electrical resistance of the resistive element is variable corresponding to a change in flexure of the flexible substrate. The flexure of the substrate, and thus the resistive element, is caused by the physical quantity to be measured with the sensor. For example, a flexure sensor can be placed in an air pathway (duct, pipe, tube) and used to measure the air flow rate (velocity) within the pathway.
Conventionally, the electrically resistive elements of flexure sensors are manufactured using a printing technique such as screen printing or by a metal deposition technique such as sputtering. However, the electrically resistive elements formed using these techniques can have inconsistent properties due to a variety of factors such as stencil accuracy, material thickness, and material composition. These factors can vary from day to day during the manufacturing process. Therefore, the response (transfer function) of any device that utilizes a flexure sensor having resistive elements created using these techniques are not typically uniform (consistent) for all sensors but rather unique for each device. In other words, a device using a resistive flexure sensor needs a way to be “calibrated” as a system to compensate for the generally loose tolerances of the sensor.
In a conventional approach, the resistive element of a flexure sensor is coupled to a biasing/scaling network configured to provide a predetermined amount of current through the resistive sensor element so as to produce a flexure-dependent variable voltage within some desired range. One or more fixed-value resistors are generally used to bias the resistive element of the flexure sensor, including a common resistor divider or Wheatstone Bridge configuration. However, changes in ambient temperature in the system can non-uniformly affect the resistance response of the resistive element of the flexure sensor and the biasing network because the temperature coefficients of the resistive flexure sensing element and the biasing network are not exactly the same. Therefore, calibration of the sensor is difficult because the biasing network cannot adequately compensate for the non-uniform variable response of the resistive element of the sensor.
Thus, a need exists for a flexure sensor having an improved biasing network for self-calibrating the flexure sensor and cancelling-out the effects of part-to-part variation and temperature-dependent shifts.